The following U.S. Patents are believed to represent the current state of the art:
4,474,572; 4,475,906; 4,484,910; 4,487,602; 4,505,710; 4,512,767;4,515,590; 4,518,387; 4,529,401; 4,529,403; 4,530,695; 4,534,759;4,547,189; 4,553,962; 4,573,970; 4,573,976; 4,578,061; 4,578,064;4,580,561; 4,592,744; 4,594,073; 4,596,558; 4,597,753; 4,600,403;4,601,708; 4,613,328; 4,620,540; 4,620,847; 4,624,660; 4,650,468;4,658,830; 4,659,326; 4,664,651; 4,664,654; 4,666,436; 4,672,967;4,681,565; 4,687,465; 4,687,467; 4,689,042; 4,699,614; 4,710,170;4,723,937; 4,735,618; 4,738,663; 4,743,234; 4,744,955; 4,745,907;4,747,829; 4,747,831; 4,753,636; 4,755,169; 4,758,227; 4,758,230;4,758,231; 4,766,908; 4,767,407; 4,767,413; 4,770,655; 4,781,683;4,781,685; 4,781,688; 4,784,640; 4,787,384; 4,787,893; 4,790,823;4,790,827; 4,795,432; 4,795,433; 4,798,587; 4,799,921; 4,804,370;4,808,169; 4,813,937; 4,813,940; 4,820,275; 4,820,286; 4,826,484;4,826,489; 4,826,490; 4,828,548; 4,832,682; 4,832,693; 4,834,704;4,834,718; 4,842,598; 4,846,811; 4,850,961; 4,850,968; 4,850,971;4,850,976; 4,850,977; 4,850,994; 4,861,338; 4,863,427; 4,863,435;4,863,436; 4,865,592; 4,874,372; 4,874,382; 4,883,466; 4,883,472;4,886,499; 4,887,998; 4,892,107; 4,892,523; 4,894,054; 4,894,055;4,898,589; 4,900,303; 4,900,307; 4,900,311; 4,902,279; 4,904,242;4,906,236; 4,908,022; 4,909,794; 4,909,795; 4,911,706; 4,913,702;4,915,702; 4,917,672; 4,919,146; 4,919,657; 4,923,443; 4,923,445;4,927,414; 4,929,237; 4,929,241; 4,931,040; 4,932,944; 4,932,946;4,932,947; 4,935,013; 4,935,014; 4,936,830; 4,941,879; 4,944,723;4,944,725; 4,946,441; 4,950,240; 4,950,241; 4,950,250; 4,950,252;4,955,866; 4,955,868; 4,955,869; 4,955,870; 4,961,728; 4,966,589;4,966,592; 4,966,593; 4,973,310; 4,973,317; 4,976,704; 4,988,335;4,988,339; 4,994,045; 4,998,921; 4,998,922; 5,000,736; 5,000,737;5,002,548; 5,007,903; 5,011,475; 5,015,240; 5,017,187; 5,019,043;5,019,044; 5,019,047; 5,019,048; 5,021,059; 5,024,665; 5,026,349;5,030,208; 5,034,003; 5,037,306; 5,037,382; 5,037,393; 5,037,400;5,041,094; 5,042,977; 5,045,066; 5,047,016; 5,049,133; 5,049,136;5,053,010; 5,053,018; 5,055,102; 5,057,086; 5,057,089; 5,059,180;5,059,185; 5,061,249; 5,061,251; 5,064,419; 5,067,490; 5,067,948;5,071,353; 5,080,104; 5,084,027; 5,084,029; 5,084,030; 5,085,640;5,085,641; 5,085,642; 5,088,986; 5,088,988; 5,092,843; 5,092,851;5,092,852; 5,092,853; 5,098,382; 5,098,400; 5,098,401; 5,102,393;5,102,397; 5,104,378; 5,104,380; 5,104,384; 5,104,385; 5,106,370;5,106,372; 5,106,379; 5,108,378; 5,108,379; 5,112,307; 5,112,316;5,114,404; 5,120,310; 5,120,314; 5,120,321; 5,122,118; 5,122,124;5,125,898; 5,125,899; 5,127,910; 5,135,507; 5,135,510; 5,137,515;5,137,516; 5,141,496; 5,143,414; 5,147,311; 5,147,326; 5,147,327;5,149,323; 5,152,751; 5,156,599; 5,160,326; 5,163,916; 5,163,917;5,163,918; 5,167,632; 5,167,641; 5,169,389; 5,169,392; 5,176,641;5,176,655; 5,176,656; 5,176,657; 5,183,468; 5,183,469; 5,188,614;5,190,526; 5,193,552; 5,195,982; 5,195,983; 5,195,985; 5,199,952;5,201,708; 5,201,710; 5,205,826; 5,205,827; 5,207,646; 5,207,699;5,209,739; 5,211,628; 5,211,629; 5,215,524; 5,215,533; 5,215,534;5,215,535; 5,215,536; 5,217,437; 5,219,338; 5,221,262; 5,222,943;5,222,947; 5,222,974; 5,224,936; 5,226,882; 5,228,883; 5,232,457;5,232,458; 5,238,654; 5,242,388; 5,242,401; 5,242,416; 5,242,420;5,246,428; 5,250,031; 5,256,152; 5,257,976; 5,261,894; 5,263,933;5,267,961; 5,267,963; 5,269,761; 5,269,762; 5,269,766; 5,273,532;5,273,538; 5,273,539; 5,273,541; 5,273,544; 5,279,554; 5,279,566;5,279,577; 5,279,579; 5,279,581; 5,279,582; 5,279,583; 5,279,590;5,282,793; 5,282,822; 5,282,827; 5,284,479; 5,290,233; 5,290,239;5,290,240; 5,290,254; 5,292,314; 5,295,963; 5,295,965; 5,295,972;5,295,973; 5,295,974; 5,295,975; 5,300,029; 5,300,030; 5,300,040;5,300,045; 5,304,137; 5,304,138; 5,306,251; 5,306,258; 5,308,332;5,311,841; 5,312,353; 5,312,366; 5,312,368; 5,312,370; 5,312,371;5,312,372; 5,314,503; 5,318,538; 5,320,609; 5,322,517; 5,324,265;5,328,475; 5,328,482; 5,328,484; 5,330,430; 5,334,149; 5,334,158;5,334,173; 5,336,180; 5,336,187; 5,336,199; 5,338,303; 5,338,311;5,342,310; 5,342,320; 5,344,407; 5,344,408; 5,346,475; 5,346,480;5,346,481; 5,348,544; 5,352,200; 5,352,202; 5,352,203; 5,354,287;5,356,387; 5,358,489; 5,360,410; 5,364,362; 5,364,370; 5,366,447;5,368,568; 5,368,570; 5,368,571; 5,370,619; 5,370,626; 5,374,250;5,378,240; 5,383,857; 5,385,550; 5,385,551; 5,385,557; 5,389,076;5,389,085; 5,391,151; 5,391,183; 5,395,317; 5,395,337; 5,399,163;5,401,246; 5,401,249; 5,401,251; 5,403,286; 5,403,287; 5,405,326;5,405,327; 5,407,436; 5,409,466; 5,411,487; 5,415,638; 5,415,645;5,415,648; 5,419,766; 5,419,773; 5,423,746; 5,425,715; 5,425,722;5,429,611; 5,429,612; 5,429,613; 5,431,631; 5,431,632; 5,433,712;5,445,618; 5,445,620; 5,451,210; 5,458,576; 5,458,580; 5,460,611;5,462,531; 5,466,223; 5,468,227; 5,474,687; 5,478,314; 5,478,316;5,478,328; 5,480,385; 5,480,387; 5,480,390; 5,482,039; 5,484,414;5,486,163; 5,486,164; 5,487,732; 5,487,733; 5,487,734; 5,489,272;5,492,536; 5,496,278; 5,501,672; 5,512,048; 5,512,050; 5,514,097;5,514,107; 5,520,639; 5,520,649; 5,522,797; 5,522,812; 5,527,283;5,527,284; 5,527,307; 5,529,189; 5,531,691; 5,531,692; 5,531,694;5,531,704; 5,531,706; 5,533,975; 5,533,984; 5,536,243; 5,536,253;5,536,257; 5,538,506; 5,538,508; 5,540,664; 5,540,666; 5,542,920;5,542,927; 5,549,558; 5,549,568; 5,549,570; 5,549,572; 5,549,708;5,558,648; 5,562,623; 5,562,624; 5,562,626; 5,562,631; 5,569,202;5,569,203; 5,573,513; 5,575,770; 5,578,011; 5,578,014; 5,578,015;5,582,591; 5,586,976; 5,591,133; 5,591,134; 5,591,138; 5,593,387;5,593,390; 5,599,309; 5,599,313; 5,599,316; 5,599,318; 5,601,532;5,601,535; 5,605,544; 5,609,577; 5,611,781; 5,611,782; 5,613,500;5,613,951; 5,613,952; 5,615,771; 5,616,123; 5,616,132; 5,616,134;5,616,135; 5,620,422; 5,620,425; 5,624,401; 5,624,405; 5,628,765;5,630,803; 5,632,730; 5,632,733; 5,634,906; 5,634,909; 5,634,937;5,637,092; 5,637,094; 5,643,220; 5,643,222; 5,647,851; 5,649,622;5,651,774; 5,653,687; 5,653,688; 5,653,693; 5,656,031; 5,658,256;5,658,257; 5,658,258; 5,658,259; 5,662,610; 5,662,617; 5,665,071;5,665,075; 5,669,889; 5,672,155; 5,672,161; 5,681,291; 5,681,295;5,688,240; 5,688,251; 5,693,016; 5,693,022; 5,693,023; 5,695,472;5,704,911; 5,704,921; 5,707,393; 5,709,662; 5,709,667; 5,709,668;5,713,866; 5,713,871; 5,713,872; 5,720,727; 5,725,498; 5,738,655;5,741,223; 5,743,879; 5,743,887; 5,743,888; 5,743,891; 5,746,718;5,749,854; 5,749,860; 5,755,692; 5,769,822; 5,769,827; 5,779,675;5,779,677; 5,779,684; 5,788,677; 5,788,713; 5,792,107; 5,792,121;5,792,122; 5,795,336; 5,797,885; 5,800,403; 5,807,334; 5,807,345;5,807,352; 5,810,775; 5,810,784; 5,817,054; 5,817,070; 5,820,602;5,823,997; 5,823,998; 5,827,293; 5,830,130; 5,836,911; 5,836,920;5,843,036; 5,843,047; 5,848,990; 5,851,197; 5,853,390; 5,853,393;5,855,839; 5,858,000; 5,865,227; 5,865,804; 5,868,711; 5,879,337;5,882,342; 5,885,257; 5,891,052; 5,891,092; 5,891,097; 5,891,105;5,897,508; 5,899,885; 5,899,886; 5,908,404; 5,908,408; 5,910,131;5,911,706; 5,919,166; 5,921,959; 5,921,960; 5,921,961; 5,921,963;5,921,964; 5,925,019; 5,928,188; 5,928,194; 5,928,205; 5,931,813;5,938,638; 5,938,639; 5,941,850; 5,944,692; 5,944,693; 5,951,522;5,954,699; 5,957,892; 5,957,895; 5,957,897; 5,960,797; 5,961,491;5,971,953; 5,976,111; 5,980,487; 5,980,488; 5,980,491; 5,980,494;5,984,899; 5,984,900; 5,989,219; 5,989,221; 5,993,417; 5,993,418;5,997,500; 5,997,511; 5,997,513; 6,001,080; 6,007,474; 6,010,486;6,010,487; 6,015,396; 6,015,438; 6,017,325; 6,022,337; 6,033,386;6,033,387; 6,036,674; 6,039,713; 6,050,974; 6,050,977; 6,056,716;6,056,724; 6,056,734; 6,063,040; 6,063,053; 6,066,115; 6,068,616;6,074,360; 6,074,369; 6,074,370; 6,077,245; 6,080,135; 6,083,199;6,083,200; 6,086,562; 6,086,569; 6,090,077; 6,090,078; 6,090,080;6,093,172; 6,099,500; 6,099,503; 6,099,504; 6,102,844; 6,113,574;6,117,112; 6,117,113; 6,126,637; 6,129,710; 6,142,972; 6,149,626;6,149,629; 6,156,008; 6,156,010; 6,156,013; 6,156,015; 6,159,161;6,159,181; 6,159,185; 6,171,284; 6,179,812; 6,183,444; 6,183,446;6,186,980; 6,192,891; 6,193,695; 6,206,856; 6,206,857; 6,210,369;6,217,550; 6,217,559; 6,221,044; 6,221,051; 6,221,052; 6,224,576;6,228,054; 6,228,055; 6,235,006; 6,241,707; 6,241,708; 6,254,575;6,254,580; 6,258,056; 6,261,264; 6,261,265; 6,267,748; 6,270,472;6,270,481; 6,273,870; 6,280,399; 6,280,420; 6,280,421; 6,283,941;6,293,925; 6,299,601; 6,309,374; 6,309,375; 6,312,409; 6,315,113;6,319,233; 6,319,234; 6,322,536; 6,325,781; 6,325,789; 6,331,173;6,332,875; 6,344,031; 6,356,783; 6,361,525; 6,368,303; 6,371,938;6,379,336; 6,387,078; 6,402,716; 6,409,701; 6,409,703; 6,409,706;6,412,490; 6,413,236; 6,413,237; 6,416,323; 6,416,497; 6,419,658;6,428,463; 6,428,517; 6,432,035; 6,432,082; 6,432,087; 6,436,068;6,440,098; 6,443,929; 6,447,480; 6,454,743; 6,458,105; 6,461,331;6,461,333; 6,468,247; 6,475,194; 6,478,780; 6,482,176; 6,485,469;6,485,474; 6,494,863; 6,500,155; 6,508,755; 6,511,454; 6,514,230;6,517,516; 6,517,517; 6,524,278; 6,527,734; 6,527,742; 6,530,896;6,530,904; 6,537,249; 6,537,252; 6,544,234; 6,547,764; 6,551,275;6,551,276; 6,551,278; 6,554,798; 6,558,351; 6,558,357; 6,565,533;6,565,538; 6,569,115; 6,572,584; 6,572,585; 6,575,939; 6,579,256;6,582,405; 6,584,910; 6,585,690; 6,585,693; 6,585,702; 6,589,158;6,592,508; 6,592,555; 6,592,556; 6,595,962; 6,599,268; 6,599,269;6,599,272; 6,605,058; 6,605,067; 6,605,073; 6,607,508; 6,607,509;6,613,019; 6,613,022; 6,616,630; 6,616,638; 6,616,639; 6,620,136;6,620,137; 6,620,138; 6,623,455; 6,623,458; 6,623,459; 6,626,864;6,629,957; 6,629,959; 6,632,198; 6,637,587; 6,638,248; 6,638,255;6,641,561; 6,645,181; 6,652,482; 6,656,164; 6,659,975; 6,659,982;6,663,593; 6,669,666; 6,673,034; 6,673,044; 6,673,049; 6,678,550;6,679,863; 6,679,864; 6,685,676; 6,685,677; 6,689,091; 6,689,106;6,689,107; 6,689,108; 6,692,470; 6,692,471; 6,699,218; 6,702,784;6,706,011; 6,706,015; 6,706,019; 6,709,416; 6,712,787; 6,712,788;6,716,191; 6,716,197; 6,716,198; 6,719,721; 6,719,728; 6,719,730;6,723,068; 6,723,072; 6,726,655; 6,726,658; 6,726,661; 6,726,662;6,730,059; 6,736,800; 6,740,059; 6,743,203; 6,749,833; 6,752,782;6,752,784; 6,752,798; 6,761,706; 6,767,336; RE 33,585; RE 34,335;RE 34,936; RE 36,398; RE 36,447; RE 37,110; RE 37,252 andRE 37,487.